In the production of optical fibers, glass fibers are heated and melt-spun, and one or more layers of radiation curable coatings are applied for protection and reinforcement. This process is called “drawing”. A structure is known in which a flexible primary coating layer is first placed on the surface of the optical fiber and a secondary coating layer with a high toughness (referred to below as “secondary coating layer”) is placed on top of the primary coating layers. Moreover, in order to make optical fiber with these radiation curable coatings applied to them suited for practical use, a structure is known in which a plurality of coated optical fibers are arranged on a flat surface, fixed with a binding material-known as a “matrix” coating, and optionally covered with further coating layers to form a structure known as a “ribbon” of optical fiber.
When the primary and secondary coating layers are formed on the surface of the optical fiber, two layers of radiation curable liquid coating are applied by means of a coating applicators, and the glass fiber is pulled at high speed and passes through a cylindrical body in a photoirradiation apparatus, at which time the composition is cured by the radiation emitted from the irradiation apparatus. When this is done, ultraviolet light is preferably used as the irradiated light which cures the layers of radiation curable coatings; mercury-vapor lamps or metal-halide lamps are primarily used as sources of this ultraviolet light.
It is generally preferred for the secondary coating layer of optical fibers to have a rigid property, with a high Young's modulus. In order to form this kind of coating layer, a radiation curable liquid coating composition with excellent curability by means of radiation is needed. If the curability is inferior, the curing of the surface layer of the secondary coating layer will be insufficient, and optical fiber with the necessary strength cannot be produced.
Furthermore, it is preferred for the secondary coating layer to have a high abrasion resistance. Since it is the outermost layer, except when overcoat layers such as up-jacket layers or ink layers are used, the secondary coating layer is destroyed and the optical properties of the optical fiber cable are harmed when there is a large amount of physical contact with the outside world and the abrasion resistance is insufficient.
As conventional secondary coating materials, liquid radiation curable compositions in which the quantity of urethane (meth)acrylate is raised and a Young's modulus which is suitable for a secondary coating layer is realized are described in Unexamined Patent Application Public Disclosure No. 2008-247981 and Unexamined Patent Application Public Disclosure No. 2007-77326.
Moreover, a liquid radiation curable composition is described in Unexamined Patent Application Public Disclosure No. 2008-249788 in which an overcoat layer with a high Young's modulus, like secondary coating layers, is formed by using ethylene-oxide-modified diacrylates of bisphenol A as an overcoat-layer-forming material which differs from the secondary coating material.
However, with the conventional liquid radiation curable compositions for secondary coating layers of optical fiber, it was not possible to obtain secondary coating layers with properties which combined high radiation curability and abrasion resistance. Therefore, it is preferred to formulate secondary coatings for optical fiber in such a way as to make them have good abrasion resistance and be good curing coatings.